Buying topsoil is something of a lottery, not least because the term ‘topsoil’ is used to describe a whole host of blended materials that are not strictly topsoils. These are often referred to as ‘manufactured topsoils’. While these do contain the basic building blocks of a topsoil in terms of their mineral particles and organic matter contents, they do not contain the numerous and diverse highly adapted soil organisms necessary to carry out the biological functions which are intrinsic to natural topsoils. “Soil-forming material” or “soil-making material” perhaps better describe these products.
A British Standard exists for topsoil (BS3882-2015), the most recent version of which was published in 2015. The standard distinguishes between ‘Multi-purpose topsoil’ and ‘Specialist topsoils’. Specialist topsoils are designed to support specific vegetation types e.g. calcareous grassland and heathland, but unfortunately none is given dedicated solely to the production of salad, vegetables, soft fruit and herbs!
Manufactured topsoils for use in growing fruit and vegetables are marketed by a large number of suppliers who almost invariably refer to the products as “meeting the requirements of” British Standard 3882-2015. Manufactured topsoils are produced most commonly by the blending of either natural topsoil or natural subsoil (most commonly the latter), sand (sometimes tailings from settlement lagoons), and green waste compost (this is produced by the recycling of green waste collections from households and landscape maintenance contractors). Manufactured topsoils are usually produced in dedicated quarries (exceptionally-agricultural processing centres).
While there is nothing false or misleading in this, it must also be recognised that these products are guaranteed only actually to meet the standard for multi-purpose topsoil: the requirements for which are very broad! For example, pH can legally range between 5.5 (moderately acidic) to 8.5 (moderately alkaline) when most crops perform best with pH in the range 6-5-7.0 (neutral to slightly acidic). Suppliers will usually be able to supply a certificate of analysis for manufactured topsoils but the requirements on testing are not that stringent (1 sample per 5,000 m3) generally, and there is no guarantee that the results accurately reflect the composition of the product you will actually receive!
Notwithstanding the above, some suppliers have recognised the need to exceed the requirements of the multi-purpose topsoil standard and manufactured topsoils for growing vegetables and fruit with added clay (most are predominantly sand), higher levels of added compost, a reduced stone content (and maximum size), and pH within a narrower range - all of which are helpful! The British Standard for topsoil provides some surety on the presence of potentially toxic elements contained within soils, although the range of elements it considers is limited to zinc, copper and nickel. Manufactured topsoils are generally supplied in 1000 litre ‘bulk bags’. Bagged soils available at garden centres usually contain sand and gravel plus a supply of organic material, which can be seaweed or even peat, rather than green waste compost.
While ‘manufactured’ (blended) topsoils should be treated with some caution, ‘screened’ mixed source soils should be entirely avoided! These are produced by the screening of demolition wastes at waste transfer sites, and include concrete and plaster, which invariably results in their being moderately (exceptionally strongly!) alkaline. Green waste compost is added to the mineral fraction which will include a high proportion of “very small” and “small” stones. Screened topsoil transforms on being wetted into a claggy-gravelly material that leaves no doubt about its source - with glass, metal and plastic pieces becoming readily apparent. The fact that these materials can meet the requirement for multi-purpose topsoil reinforces the fact that the standard is far too broad! These are more often sold as ‘economy’ manufactured topsoil for general landscaping purposes, and only the most unscrupulous supplier would claim they are suitable for growing fruit and vegetables!
“As-dug” natural topsoils are excavated from greenfield sites e.g. housing and industrial developments, where they are ‘surplus’ to requirements. The availability of “as-dug” natural soils has diminished in recent years as more and more development takes places on brownfield (industrial) land, and because planning permissions for greenfield sites require topsoils to be retained for landscaping purposes. However, when available “as-dug” topsoils benefit from being unprocessed and retain some vestige of their original structure in the form of aggregates, organic matter (which is of far better quality than that offered by the addition of green waste compost), and functioning (albeit suppressed) microbial biomass. While “as-dug” topsoils can be imperfect and contain stone, clods, vegetation remains, roots and blocks of subsoil - they will in most cases (not all) be more suitable for growing vegetables and fruit than manufactured topsoils, sold for this purpose.
The most important consideration in deciding whether or not to use “as-dug” topsoil is its texture which should be either sandy or loamy. A relatively high clay content (up to 35%) can be permissible if mirrored by a high organic matter content (in excess of 8%). We’ve found this link very helpful as providing an explanation of how to determine soil clay content!